We continue to investigate venous samplings of the pituitary venous drainage to aid in the diagnosis and treatment of patients with Cushing's syndrome. Over 150 patients have now received bilateral simultaneous inferior petrosal sinus (IPS) sampling. The results indicate that 1) procedure can be performed successfully in all patients with Cushing's syndrome (successful sampling has been performed in over 99% of the patients in whom it has been attempted); 2) the procedure distinguishes patients with ectopic ACTH secretion from those with pituitary adenomas with greater than 99% accuracy; 3) if one adds CRF stimulation to the diagnostic procedure, a diagnostic accuracy of 100% has been achieved in over 100 patients with Cushing's syndrome; 4) IPS sampling successfully determines which side of the pituitary gland microadenomas reside in patients with Cushing's disease with 85% accuracy; and 5) unilateral inferior petrosal sinus sampling, which is commonly used clinically, is frequently misleading. Repeat transsphenoidal surgery is successful in irradicating the hypercortisolism of Cushing's disease in about 60% of patients. This therapy for patients with Cushing's disease after previous pituitary surgery has not previously been examined. The CRF stimulation test and the dexamethasone suppression test are equally accurate in determining and distinguishing patients with Cushing's disease from those with ectopic ACTH secretion in Cushing's syndrome. Both have approximately 10-15% diagnostic inaccuracy, when used alone, in our experience. CT scanning of the pituitary gland is normal in 70% of patients with Cushing's syndrome.